What is the best evidence for Jupiter's and Saturn's interiors being
composed of hydrogen and helium?
A)their magnetic fields
B) their observed spectra
C) their sizes
D) their infrared radiation
E) their densities

Hello again, player55!

The correct Answer is E) their densities.

From the website of grad student Nicholas James Rattenbury at the
University of Auckland, New Zealand, Lecture #6:
"Interior of Jupiter
We can only infer Jupiter's internal composition from theory. We see
no solid surface, so we must rely on what we know about matter and
gravity to predict the interior.

The density of Jupiter is much less than the inner planets:
1.3gcm¯³(þH2O = 1gcm³)
             ==> Bulk of planet is made of light elements.
- Huge mass gives rise to large gravitational field. This pulls the
atmosphere inwards and compresses it. Density increases with depth.
- At about 10000 km down in the atmosphere, the pressure is so great
that the gas changes to liquid.
- Further down still, the pressure is great enough that metallic
hydrogen forms...

The density of Saturn is about 0.7gcm¯­­­³, less than that of water.
             ==> the composition of Saturn is mostly hydrogen
                 and hydrogen rich compounds.
- As does Jupiter, Saturn radiates more heat than it receives from the
Sun. This indicates the presence of an internal heat source. However,
Saturn's internal heat comes from the gravitational release from
helium condensing out of the atmosphere and falling to the planet
centre."
http://moa.scitec.auckland.ac.nz/~nick/Lectures/lecture6.pdf


From the UCL Hot-Star Group's website on the geology of the planets: 
"Saturn, like Jupiter, is composed primarily of hydrogen and helium.
Current models of the interior indicate that below the relatively
thin, opaque cloud layers is a large, clear hydrogen-helium
atmosphere. The cloud layers are similar to those on Jupiter: ammonia
ice in the upper layer, crystals of a compound of ammonia and hydrogen
sulphide in the middle layer, and water ice in the lower layer. As on
Jupiter, the density of the atmosphere gradually increases toward the
surface, and the gas transforms into a liquid. At the centre, the
hydrogen becomes metallic; Saturn, however, with its lower mass and
internal pressure, has a much smaller region of metallic hydrogen than
Jupiter."
http://www.star.ucl.ac.uk/~idh/geology/saturn.htm
http://www.star.ucl.ac.uk/~idh/geology/g3622.htm


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I hope that this Answer has provided you with exactly the information
that you needed!
   
Regards,   
   
aceresearcher